@PRR Man posted:In my pursuit for more accurate model operation. I'd like to ask the trainmen here to comment or verify my findings.
In determining the amount of weight a single unit engine can handle on the road. My research came up with these figures:
Using 1750hp as found in a F7 or GP9 (567c engine) the stated tractive effort (TE) is 64,750 ftlbs. Using a 72% efficiency rating the TE at the rails is about 46,620. I have found specs that state it takes 2-5 lbs of TE per ton to start a train. Since I model the mid-50s with a lot of friction bearing rolling stock, I'll use the high number. Add +10 lbs of TE per ton to accelerate the train, and +20 lbs per ton for a 1% grade. That's a total of 35 lbs TE per ton of train.
That provides 1332 tons of TE. At nominally 50 tons per car, that's 26.6 cars?! It doesn't sound correct. Then I realized that TE is reduced when speed increases. So at 25 mph the figure becomes 9.2 cars. At 40 mph it becomes 5.8 cars.
Am I anywhere near right in where I'm going with this? Thanks for your input.
First, drag, or friction, is highest when the train is stopped, until it gets to a speed where wind resistance takes over, the speed of which is probably quite high for a train.
If you pull a car with a constant force, you will accelerate until the drag increases to match the pulling force.
I think there are two problems with your assertion. You state it takes more TE to accelerate a static load. This is not true. You also estimate quite a high number for “friction bearings”. I doubt that they are the 100-200% increase in friction you estimated.
